The present invention relates to the low temperature thermal upgrading of nickel-containing lateritic ores to provide concentrated metal values amenable to magnetic separation. More particularly, the invention relates to a process whereby lateritic ores undergo heat treatment to induce selective reduction to metallic values followed by concentration of the metals. Even more specifically, the invention provides a process for the thermal upgrading of nickel-containing ores with a high iron/nickel weight ratio.
Lateritic nickel ores are of two types, referred to as saprolites and limonites. Saprolites consist mainly of hydrated magnesium, iron and nickel silicates with a nickel content of about 2-2.5%; limonites consist mainly of hydrated ferric iron oxide, with a nickel content of about 1-1.5%. These ores also contain minor amounts of cobalt. In contrast with sulfidic nickel ores, which are amenable to concentration by physical techniques, lateritic ores are characterized by wide dispersion of the oxidic nickel values throughout the ore in the form of solid solution in the ore minerals. Consequently, the hydrometallurgical and pyrometallurgical processes which are currently practiced to recover nickel from lateritic deposits must handle all of the ore through the various stages of the process. It is therefore quite desirable to develop techniques which can render lateritic ores amenable to concentration prior to leaching or smelting to reduce the cost and environmental hazards associated with the handling of unwanted material. The lower the nickel content of the ore, e.g. limonites, the more desirable are these techniques.
One technique which has been proposed in the past to render a lateritic ore amenable to concentration by physical means is thermal upgrading. In this process, the ore is subjected to reduction to form ferronickel particles. (Cobalt, if present in the ore, is also reduced, and reports to the ferronickel particles. Accordingly, all references herein to metallics resulting from the reduction of lateritic ores should be understood as an iron-nickel-cobalt containing alloy.) Concentrating agents are added to the ore to enhance the growth of the ferronickel particles to a size which makes them amenable to concentration by comminution and magnetic separation.
Numerous attempts have been made to develop an effective thermal upgrading process. None of these, however, has been shown to be commercially feasible. Furthermore, most of them have preferentially addressed the treatment of the saprolitic, high grade ores. As a result, there are no thermal upgrading processes applicable to the lower grade limonite or blends of limonite and saprolite with high iron/nickel weight ratio.
U.S. Pat. No. 3,388,870 to Thumm et al discloses a process wherein the ore is pelletized with concentrating agents, including a sulfur-bearing material, and a reagent from the group consisting of alkali and alkaline earth metals. The pellets, along with a reducing agent, such as reducing gas or fuel oil, are charged into a reacting vessel preferably at 950.degree.-1150.degree. C. In addition, carbon reductant may be incorporated into the pellets. Temperature, retention time and atmosphere are controlled so as to reduce substantially all the nickel to metallic nickel and substantially all the iron to wustite (nominal FeO), with a limited amount to metallic iron.
U.S. Pat. No. 4,490,174 to Crama et al discloses a process whereby lateritic ore is reduced at 920.degree.-1120.degree. C. in a CO/CO.sub.2 atmosphere in the presence of a sulfur compound concentrating agent to produce a ferronickel concentrate. Crama et al have, in a sense, improved upon Thumm et al by eliminating the need for an alkali or alkaline earth metal concentrating agent. However, Crama et al employs a gaseous reduction reaction which requires such an overwhelming amount of gas to ore ratio and has such a slow reaction time, as to make this process commercially impractical. Though Crama et al recognize the potential for the use of solid reductant, they fail to develop the specific conditions for the effective use of this type of reductants.
Other existing problems in the field of thermal upgrading technology include the control of the reduction reactions. This is of critical importance in the treatment of ores with high iron/nickel weight ratios. Since iron is present in amounts as high as 40 times that of nickel, it is desirable that substantially all the nickel be reduced to metal while only a minor proportion of the iron is reduced to metal. In addition, it is difficult to prevent the appearance of unwanted iron oxide phases. It is desirable to have as an end product metallic ferronickel and non-magnetic wustite, which will easily separate from each other. However, in the treatment of these type of ores, magnetite may be formed by disproportionation of the wustite during cooling of the thermally upgraded material, having the result that in the subsequent separation the magnetic fraction becomes diluted with this contaminant.
Most of the previously proposed thermal upgrading processes recommend temperatures which are above the so called "softening" temperature of the ores in order to achieve the desired growth of the ferronickel particles. However, above this temperature the ore becomes sticky. Consequently, the ore agglomerates, pellets or briquettes, sinter to each other and form accretions on the furnace walls. Coating of the agglomerates has been suggested as a solution to this problem. However, this adds an additional operating step. In contrast, the present invention practices a low temperature type of thermal upgrading, wherein "low temperature" is defined as the maximum temperature compatible with the avoidance of stickiness among the agglomerates.
Another variation of thermal upgrading is the process currently practiced by Nippon Yakin Kogyo Co. Ltd. at Oheyama, Japan as described by Arai et al in "An Economical Process for Stainless Steel Production from Nickel Ores", Proceedings of the International Symposium on Ferrous and Non-Ferrous Alloy Processes, Hamilton, Ontario, Canada, Aug. 26-30, 1991. This process is related to the Krupp-Rehn process for direct reduction of iron ore in rotary kilns. It involves the semi-fusion of the ore to provide the conditions to grow the ferronickel to millimeter size particles. This process is limited to a certain type of saprolitic ores with a low iron/nickel weight ratio. In addition, the semi-fused ore causes the formation of rings on the kiln walls. This results in substantial kiln downtime.
It is thus an object of the present invention to provide a process for the low temperature thermal upgrading of lateritic ore which produces a high grade nickel concentrate at high recovery for a low cost.
It is a further object of the present invention to provide a process which utilizes a minimum of reagents and has a relatively fast reaction time.
It is a still further object of the invention to provide a process which can achieve high nickel grade and high recovery by controlling the degree of iron reduction and metallic particle growth and preventing the formation of unwanted constituents.
It is an additional object of the invention to provide a process which will be effective for both limonite and blends of limonite and saprolite.